Sample preparation for urine THC-COOH in GC/MS
First I would like to emphasize that the world of chromatography is quite recent for me ( 1 year).
I would like to analyze THC-COOH in urine with a GC/MS but I have some issues below 100 ug/L. I can detect THC-COOH > 100 ug/L ( according to the immunoassay) but nothing below in urine samples.
All the solutions are new and analytical grade.
To explain this lack of sensitivity, I suspect an issu with my sample preparation or the injection.
Since I am not an expert at all in this field, I hope someone could tell me what am I doing wrong.
Here is my sample preparation:
1 mL urine
Add 125 uL KOH 12N and heat 15 min at 60°C for the hydrolysis
Cool the sample and add 450 uL of acetic acid in order to obtain a pH 4-5
Add 4 mL 1-chlorobutane
Vortex 2 minutes and centrifuge
Collect the organic phase into a 5 mL glass tube without taking any of the aqueous phase
Evaporate to dryness at 37°C under slow nitrogen flow
Residue is resuspended into 75 uL of BSTFA ( 1%TMCS) and heated at 60°C, 30 min for derivatization.
The solution is eventually transfered into a GCMS vial and 2 uL are injected into the GC/MS.
GC: Agilent 7890 and MS: Agilent 5977
Injection temperature: 250°C
Injection mode: splitless
The aquisition is not made in SIM mode but SCAN mode.
EM setting is set on gain factor
Autotune was recently made and liner changed.
Any help would be greatly appreciated